Process for producing a corrosion and wear-resistant oxide layer with locally reduced layer thickness on the metal surface of a workpiece

ABSTRACT

The invention relates to a method to produce an oxide coating that protects against corrosion and wear on the metal surface of a workpiece, more specifically, for the metal surface of a body for accepting a valve, wherein with the use of a cathode, the workpiece, as an anode, is subjected to constant or pulsating direct current in an electrolyte, with an auxiliary anode, which is resistant to the electrolyte, being attached to the workpiece before the workpiece is dipped into the electrolyte, which discharges the anodizing current at locations where no anodized coating is required or only a limited anodized coating is required.

This is a national stage application of PCT/EP95/04544, filed Nov. 18,1995.

FIELD OF THE INVENTION

The invention relates to a method to produce an oxide coating on themetal surface of a workpiece that protects against corrosion and wearand has a selectively reduced coating thickness and a device to carryout the method.

BACKGROUND OF THE INVENTION

It is already known from the technical literature that anodic oxidationmay be carried out to produce an oxide coating which protects againstcorrosion and wear on the metal surface of certain metal alloys. Thisprocess has great technical significance for the treatment of metalsurfaces which may be coated in this way with an oxide coating whichprotects against corrosion and wear (for more about this, compareKraftfahrtechnisches Taschenbuch, Bosch, 21st Edition). Anodic oxidationis utilized, above all, in the surface treatment of an aluminumworkpiece to reinforce the natural oxide film and thus to achieve aso-called anodized coating.

An aluminum part is wired as an anode and, together with acounter-electrode, receives a constant or pulsating direct current in anelectrolyte. The transformation (oxidation) of metallic aluminum toaluminum oxide forms a coating on the surface, which protects againstcorrosion and wear. The structure and thickness of the coating areessentially a function of time, temperature and current density.

As a rule, the aluminum oxide coating is very hard and brittle, andtherefore it cannot be prevented from chipping, for a correspondingexterior influence of force on the anodized coating, which isundesirable in certain applications.

It is also known that the anodizing of special locations of a componentmay be prevented or limited to a minimum by means of masking with aninsulating lacquer or with protective screens.

However, these methods are very costly for industrial scale manufactureand cannot be automated for the application described, or may beautomated only at great cost.

OBJECT OF THE INVENTION

Consequently, the purpose of the invention is to indicate a method and adevice of the aforementioned type, which do not have the describeddisadvantages and which permit, on one hand, an oxide coating whichprotects against corrosion and wear at the desired locations of themetal surface but, on the other hand, limits or prevents the build-up ofan anodized coating at locations of the workpiece where a chipping-offof the anodized coating must be prevented under all circumstances.

SUMMARY OF THE INVENTION

This problem is solved in that an auxiliary anode that is resistant tothe electrolyte is attached to the workpiece, to which the currentacting upon the workpiece may be partially dissipated. Consequently, itis possible to limit the thickness of the oxide coating in said region,or even to prevent same. Consequently, mechanical stress acting upon thenonanodized locations may be absorbed without danger. A device to carryout the method comprises a cover which is adapted to be placed on theworkpiece and to which is attached at least one auxiliary anode, whichis connected to a direct-current source in a circuit which iselectrically parallel to the workpiece.

Additional objectives, features, advantages and possibilities for use ofthe present invention are obtained from the following description of anembodiment.

BRIEF DESCRIPTION OF THE DRAWING

In the drawings,

FIG. 1 shows the principle construction to produce an oxide coating toprotect against corrosion and wear on a metal surface of a workpiece;

FIG. 2 shows a direct-current source with a separate rectifier toregulate the current in a device according to FIG. 1; and

FIG. 3 shows a direct-current source with a variable resistor toregulate the current in a device according to FIG. 1.

DETAILED DESCRIPTION OF THE DRAWING

FIG. 1 shows the principle construction to produce an oxide coating toprotect against corrosion and wear on the metal surface of a workpiece1, which is dipped in an anodizing electrolyte. The workpiece 1 consistsof an aluminum alloy and, as a positive pole (anode), is connected to adirect current source 10 by means of an electrical lead 7. The negativeelectrode (cathode 5) consists, e.g., of a lead plate which, in turn, isconnected to the direct current source 10 by means of an electrical lead8. The workpiece 1 is covered in the region of the openings 6 (drilledholes for accepting valves), which must not be anodized, with aplate-shaped protective cover 4. The auxiliary anodes 3 are attached toa protective cover 4 such that they are positioned in the region of theopenings 6 and are connected to the anodizing electric circuit by meansof the electrical lead 9. Thus, they can dissipate the anodizingcurrent, preventing the anodizing current from spreading through thedrilled channel holes 11 into the drilled holes for accepting valves(openings 6).

In addition to the details shown in FIG. 1, it is also possible,however, to regulate the current to be dissipate by means of theauxiliary anodes by means of a separate rectifier 12 or by means of avariable resistor 13 on the lead 9 (see FIG. 2 and FIG. 3).

For this purpose, the positive pole of the rectifier 12 is connected tothe lead 9 leading to the auxiliary anode 3, while the negative pole ofthe rectifier 12 is connected to the lead 8, which is connected to thecathode 5.

When a variable resistor 13 is used, it is wired between the positivepole of the direct current source 10 and the lead 9 leading to theauxiliary anode 3.

The auxiliary anodes should consist of a material (e.g., platinum,platinized titanium, graphite, conductive plastic) which is resistantunder anodic polarization in an anodizing electrolyte (e.g. sulfuricacid).

The material of the cover on which the auxiliary anodes are attachedought to consist of an electrically nonconductive material (e.g.plastic) or of an electrically conductive material (e.g., titanium)which becomes electrically passive during anodic polarization in ananodizing electrolyte, and it likewise must be resistant to theanodizing electrolyte.

The plate-shaped cover 4 which, on one hand, assumes the function of abody which closes the openings 6 with respect to the electrolyte 2, alsoserves, on the other hand, to position the auxiliary anodes 3 on theworkpiece 1 during the anodizing process in the electrolyte 2.Consequently, the auxiliary anodes 3 are a component of the cover 4 andare carried by the cover 4. It is preferable for the cathode 5 to bemanufactured of lead or aluminum, and it is arranged in the anodizingelectrolyte. The workpiece 1 forms the body of a valve receiver which,as a monolithic block, features several channels and openings. Afteranodizing of the metal surface has been carried out, the openings 6 ofthe workpiece 1, closed by means of the cover 4, accept several valveswhich must be inserted at a relatively large insertion force and which,as a result of an anodized coating which is reduced by means of theauxiliary anodes 3, may be pressed into the openings 6 without wear byfriction. Consequently, possible contamination of the channels resultingfrom chipped-off pieces of coating is prevented. This also rendersunnecessary the otherwise necessary finishing measures to the workpiece1, which would require, more specifically, machining and costly rinsingprocesses of the entire channel system. An additional advantage of theinvention is obtained by means of an exact maintaining or control of thedimensional tolerances of the workpiece 1, since, on one hand, openings6 worked to a precise fit in order to accept valves do not requireanodizing or only require slight anodizing; on the other hand, thepossibility to control the anodizing current permits variousconfigurations, in accordance with need, of the coating thickness of theremaining metal surface, which is covered by an anodized coating. Thenumber of required auxiliary anodes 3 is set to the number of openings 6of the valves to be inserted. Since, as a rule, the openings 6 arearranged in two parallel rows, a plate-shaped cover 4 results, withauxiliary anodes 3 likewise parallel to each other in rows.

We claim:
 1. A method for producing an oxide coating on a metal surfaceof a workpiece, which comprises:providing a cathode in an anodizingelectrolytic bath; providing said workpiece, as an anode, in saidanodizing electrolytic bath; providing at least one auxiliary anode insaid anodizing electrolytic bath in a vicinity of a portion of saidworkpiece where said oxide coating is undesirable, attaching said atleast one auxiliary anode to a cover that is electrically passive insaid electrolytic bath, and placing the cover on said workpiece, wherebysaid portion of said workpiece is closed by said cover, but said atleast one auxiliary anode is not in contact with said workpiece;subjecting said workpiece and said cathode to a first direct currentsource; and preventing formation of said oxide coating in said portionof said workpiece by subjecting said at least one auxiliary anode to thefirst direct current source or to a second direct current source.
 2. Themethod of claim 1, wherein the at least one auxiliary electrode isconnected to a second direct current source and at least one of saidfirst and said second direct current source provides a pulsatingcurrent.
 3. The method of claim 1, wherein a single direct currentsource is utilized, and said at least one auxiliary anode is wired inparallel with said workpiece.
 4. The method of claim 1, wherein saidelectrolytic bath comprises sulfuric acid.
 5. The method of claim 1,wherein said cathode comprises a lead plate.
 6. The method of claim 1,wherein said at least one auxiliary anode comprises at least one ofplatinum and platinized titanium.
 7. The method of claim 1, wherein theportion of said workpiece where said oxide coating is undesirable is anopening, and said at least one auxiliary anode is aligned with saidopening.
 8. The method in accordance with claim 1, wherein the directcurrent applied to the auxiliary anode is controlled by means of arectifier.
 9. The method in accordance with claim 1, wherein the directcurrent applied to the auxiliary anode is controlled by means of avariable resistor.
 10. A device for producing an oxide coating on ametal surface of a workpiece, which comprises:a cover which iselectrically passive in an electrolytic bath, and is adapted to beplaced on a predetermined portion of the workpiece where said oxidecoating is undesirable; at least one auxiliary anode, attached to saidcover in a vicinity of said predetermined portion; a firstdirect-current source in a circuit which is electrically parallel to theworkpiece, and connected to said auxiliary anode; and a connection fromsaid first direct-current source to said workpiece or a seconddirect-current source connected to said workpiece, wherein said cover isconfigured to electrically separate said workpiece from said auxiliaryanode when the cover is placed on the predetermined portion of theworkpiece, and said cover facilitates reduction of said oxide coating bysaid auxiliary anode at said predetermined portion.
 11. The device ofclaim 10, wherein the cover is adapted to be placed on a predeterminedarea which is an opening in the workpiece.
 12. The device of claim 10,wherein the cover has essentially the form of a level plate.
 13. Thedevice of claim 12, wherein said second direct-current source isprovided with a separate current-controlling rectifier in the auxiliaryanode electric circuit.
 14. The device of claim 12, wherein said seconddirect-current source is provided with a current-controlling variableresistor in the auxiliary anode electric circuit.
 15. The device ofclaim 12, wherein the cover accepting the auxiliary anode consists oftitanium.
 16. The device of claim 12, wherein the cover accepting theauxiliary anode consists of plastic.
 17. The device of claim 10, whereina single direct current source is provided.